a(n) = 9*7^n-1.

Terms

a(0) =8a(1) =62a(2) =440a(3) =3086a(4) =21608a(5) =151262a(6) =1058840a(7) =7411886a(8) =51883208a(9) =363182462a(10) =2542277240a(11) =17795940686a(12) =124571584808a(13) =872001093662